Effect of yarn,machine and knitting process parameters on the dynamics of the circular knitting needle

Abstract

Stitch formation during the knitting process is dependent on the dynamic movements of needles. The movement of the needle is subjected to many different factors during loop formation, such as machine, yarn and knitting process parameters. In particular, machine speed, yarn tension and fiber type are the vital parameters of needle movement. This study was conducted in order to investigate the effect of these parameters on needle dynamics during one knitting cycle. High-speed camera technology was used for the first time to collect data regarding needle movement on a circular knitting machine of E 24 and 30” in diameter.

The results showed that the needle displacement in both x and y coordination did tend to increase as the yarn tension increased. For all situations, the elastane yarn was an influential parameter on the needle displacement, although the change in its input tension was not a significant parameter. It was also found that during the production with 100% ring cotton, there was a tendency for the needle to follow the cam path when the machine speed was increased from 15 to 20 rpm. Finally, the data obtained for knitting with the textured polyester yarn showed that the needle movement was highly dependent on yarn input tension.

Keywords

needle displacement high-speed camera fiber type machine speed yarn tension

During the process of loop formation, an interaction takes place between various yarn variables (rigidities, coefficient of friction, diameter, etc.), knitting machine variables (speed, diameter, machine gauge, cam shape, needle and sinker shape, etc.) and knitting process variables (yarn input tension, fabric take down tension, cam setting, etc.).¹

The majority of the manufacturers would like to produce their products with high speed but without any faults in the process. This is achieved to some extent with the advance in many industrial areas, especially machinery and computer technology. However, it has been realized that it is impossible to increase the production speed infinitely without considering the textile materials used on the machines due to the material-related problems occurring in the production.² Increasing the speeds of weft knitting machines will consequently enlarge the reaction forces between knitting elements and then cause their attrition.³

Also yarn tension is a key parameter on the force between knitting elements. In the loop formation zone, yarn tension induced by applied forces and yarn–metal contact builds up in the new loop as it is formed. The tension in each arm of the new loop determines the yarn force exerted on the hook and contributes to the reactive force at the butt. This variable yarn force is operative during loop formation and is influenced by yarn physical characteristics and the geometry of the system.⁴ In addition to that, the yarn input tension plays an important role in determining the value of loop length for at any cam section.¹ The variation in loop length, which occurs when knitting under different conditions of yarn input tensions, is attributed possibly to the following factors:⁵

needle fling-changes in the needle direction due to inertia forces after the knitting point has been reached;

yarn extension at high knitting tension;

robbing back.

Hence, tension control should be properly maintained and optimized during the knitting process.²

In addition to above findings, yarn properties determining needle displacement during the knitting process may be summarized as follows:^6–11

finer yarns were more abrasive than the coarser ones;

lower twist levels resulted in more abrasion;

Open-end yarn is more abrasive than ring yarn due to belt fibers;

rotor yarns produced with higher rotor speeds were more abrasive;

yarn contamination as a major contributor to needle wear;

ring spun yarn that was produced from dirty cotton was more abrasive;

increase in wax levels did not affect abrasiveness;

excessive amount of fall out was seen when using ring spun yarn;

oxides or silicates on the surface of a spun yarn result in abrasion of the needle material;

synthetic fibers that contain matting agents such as titanium dioxide can induce needle wear.

A literature survey also showed that there are many studies that focus on the forces on needles and cams for circular knitting machines. They have generally studied the theoretical models, working with strain gages or a computer-aided design system.^3,12–20

Wray and Burns^12,21,22 designed an impact transducer for measuring the rapid force fluctuations existing between the needle and stitch cam on a 10 inches diameter, 18 gauge circular knitting machine having one yarn feeding station with a standard 0.44 mm thick latch needle. They used semi-conductor strain gauges so that not only needle bounce on the cams can be visualized but also their duration and number could be recorded. The effects of machine speed, take down tension, input yarn tension, cam angle, needle thickness, mass and butt type, clearance between cylinder and cams were also investigated.^14,23–26

Ghosh and Banerjee¹ highlighted the role of the dynamic geometry of the knitting zone and the dynamic equilibrium of forces on the needle hook as well as the importance of cast off loops hanging around the yarn, pulled in by needles inside the knitting zone, on the loop formation process. However, in the study they worked with two different hand-driven small diameter circular machines with linear cams.

Long²⁰ analyzed cam to needle impact forces and the shape of the impact pulse in weft knitting using the mechanical model of viscously damped free vibration with one degree of freedom. The induced equations predict that the peak impact force and pulse frequency are directly proportional to the product of the natural circular frequency and the mass of the needle. However, the experimental machine was modified on the basis of a 7 inch diameter circular knitting machine and was fitted with 0.45 mm thick needles. Also, the peripheral speed of the needle cylinder was adjusted to 0.85 m/s during the test. During the experiment 45° cam and semi-conductor strain gages were used for detecting vertical and horizontal impact forces.

Oldman et al.²⁷ described an investigation into the suitability of three non-linear profiles for inclusion in a computer-aided design system for a 30 inch diameter machine running at 40 rpm. They evaluated the profiles according to the various knitting and dynamic requirements. The acceleration characteristics of cam profiles and the characteristics of reaction forces between the cam and the needle were compared and analyzed by Song et al.³ with the aid of a computer-aided design system. MacCarty et al.⁴ developed a general model for mechanical forces that arise in normal operation of a latch needle cam system. The model can predict mechanical forces for any specified cam profile and for the most common needle and cam arrangements.

As can be seen from the literature above, surprisingly enough, there is no experimental study that has been carried out on an industrial knitting machine that takes all the parameters mentioned above into account. Accordingly, this study was conducted in an attempt to investigate real-time needle movement on an industrial circular knitting machine and, in doing so, the effect of machine speed, yarn tension, yarn type and fiber type, including elastane, on the impact and resistance forces on needles was studied using a high-speed camera, which was a novel approach for such studies.

Material and method

For the work under discussion, a Beck Gmbh E 24 30” knitting machine, equipped with Groz-Beckert needles, with 90 positive feeding systems, was employed. For yarn cost purposes, only 24 feeders out of 90 were used throughout the study. Before recording any data at any machine speed, the machine ran at 10 rpm for 30 minutes to make it heat up to normal working conditions.

Also, for the study, yarn type, machine speed and yarn input tension were selected as the variables; the design of the experiment was conducted such that 24 different situations were investigated. In one set of these trials, for example, the yarn input tension was changed whilst the other variables were kept constant. Thus it could be possible to discuss the possible effects of various knitting parameters on needle movement under commercial conditions. Furthermore, for each case, the take up tension became the only parameter that was kept constant. Table 1 presents the details of the parameters taken into account for the design of experiment.

Table 1.

Knitting parameters for the experimental study

Fiber type	100% ring cotton, 100% textured polyester
Machine speed (rpm)	15 and 20
Yarn tension (cN)	3, 6 and 9
Elastane tension (cN)	0, 3, 6 and 9

Yarns (100% cotton ring spun and 100% textured polyester filament), both with and without 33 dtex elastane yarn, were employed for the work. These yarns were chosen as they were commonly used in the industry. The physical properties of the yarns are summarized in Table 2.

Table 2.

Properties of the yarns

Yarn properties	Ring cotton	Textured polyester
Yarn count	Ne 50/1	110 dtex
Elongation at break	6.33%	21.13%
Yarn tension at break	211.24 cN	431.18 cN

An Olympus i-SPEED 3 high-speed camera was employed to collect data. In addition to that, a specialized sensor was used so that the Olympus i-SPEED 3 could capture frames of video at high speeds. However, when the cam arrangement was kept as it was, it was impossible to capture a meaningful movie because the distance between the two adjacent cams was too close. Therefore, to be able to investigate the needle movement at the knock over position, a hole of 5 mm × 20.5 mm × 15 mm was drilled in the outer wall of a cancelled cam system using computer numerical control (CNC; see Figure 1), such that no deformation occurred on the cam segment itself. The camera was adjusted to 20.000 frames in order to obtain the best possible sharpness and resolution.

Figure 1.

Detailed photos of the cam system.

The i-SPEED 3’s image sensor is capable of increasing its dynamic range by detecting light in a non-linear fashion. The way the sensor detects the light permits each pixel to individually reduce its sensitivity with increasing light, and it also allows the sensor to gather image features in very bright and dark areas simultaneously.²⁸ Accordingly, the lighting system together with the camera was positioned in the way given shown in Figure 2, and thus a better lighting with higher sensitivity could be obtained.

Figure 2.

Position of the camera and light during recording data.

For the data evaluation, the Olympus i-SPEED 3 software suit was utilized. In order to convert the images from pixels to units of inches, meters, millimeters, etc., it was calibrated according to distance between the stitch and clearing cam, which was 4.8 mm (see Figure 3).

Figure 3.

Evaluation of the needle movement.

For easy tracking of needle movement, firstly the butt of a chosen needle was painted with oil resistance white dye. Then two additional points were marked on the needle, which were close to the upper and lower edges of the needle butt. The distance between the marked points and the very end points of the butt edges of the needle was measured with the relevant tool of the program.

Manual tracking requires the user to manually step through the video one frame at a time, using the mouse to select the location of the feature in each frame.²⁸ In the work, however, with the help of the points marked on the chosen needle, the whole stitch forming process was followed at intervals of every five steps. Accordingly, not only both ends of the needles were observed, but also with reference to the selected points, both the observations from clearing and stitch cams were also made. In Figure 4, a typical representation of the needle movement within the cam track is shown. Figure 4(a) represents the first contact point of the needle to the cam, while Figure 4(b) shows the second contact point. After recording was finished, the tracked points were exported to MS Excel for further evaluation and calculations.

Figure 4.

(a) The first contact point to the cam. (b) The second contact point to the cam.

Both vertical and horizontal velocities of the needle were calculated using Equations (1) and (2):

v_{x} = \frac{Δ x}{Δ t}

(1)

v_{y} = \frac{Δ y}{Δ t}

(2)

The acceleration of the needle was calculated using Equation (3):

a = \frac{Δ v_{y}}{Δ t}

(3)

The statistical evaluation of the data obtained was performed using the SPSS 18 software package. To do so, a one-way analysis of variance (ANOVA) and t-test methods were employed and the factors were considered to be significant at a p-value less than 0.05.

Results

In the study, the effect of fiber type, machine speed and use of elastane on the needle movement was investigated. The graphs related to the needle dynamics are given in Figures 5 –14. Also, the statistical evaluation of the results regarding the needle displacement can be seen in Table 3. In the following sections of the paper, the relevant parts of these results are discussed in detail.

Figure 5.

Effect of yarn tension on needle displacement behavior and velocity at 15 rpm during production with ring cotton yarn.

Table 3.

Statistical evaluation of the results

	Effect of machine speed
	Ring cotton				Textured polyester
	t	p	t	p	t	p	t	p
Without elastane	22.000	0.000	5.000	0.001	–1.27	0.24	–0.286	0.786
With elastane	–1.49	0.148	1.199	0.241	1.674	0.106	0.721	0.477

	Effect of elastane
	15 rpm				20 rpm
	Displacement in x direction		Displacement in y direction		Displacement in x direction		Displacement in y direction
	t	p	t	p	t	p	t	p
Ring cotton	–2.657	0.012	–3.803	0.001	–5.586	0.000	–5.617	0.000
Textured polyester	–12.134	0.000	5.400	0.001	–2.121	0.041	–1.315	0.197

	Effect of fiber type
	15 rpm				20 rpm
	Displacement in x direction		Displacement in y direction		Displacement in x direction		Displacement in y direction
	t	p	t	p	t	p	t	p
Without elastane	2.403	0.043	3.965	0.004	1.109	0.299	0.555	0.594
With elastane	9.827	0.000	11.000	0.000	6.378	0.197	7.979	0.010

	Effect of yarn tension
	15 rpm				20 rpm
	Displacement in x direction		Displacement in y direction		Displacement in x direction		Displacement in y direction
	F	sig	F	sig	F	sig	F	sig
Ring cotton	325.000	0.000	400.000	0.000	400.000	0.000	400.000	0.000
Textured polyester	13075.000	0.000	2275.000	0.000	3100.000	0.000	1300.000	0.000

Effect of yarn tension and machine speed

Ring cotton yarn

At the knitting point, the needle displacement in both x and y coordination did tend to increase as the yarn tension increased. The impact of yarn tension on the displacement was more significant for 3 and 9 cN tensions, whilst the values obtained for 6 and 9 cN were close to each other. The similar tendencies were observed when working at 15 and 20 rpm machine speeds. When the speed was 15 rpm, the needle bounced to 1.5 mm in the x direction and 0.4 mm in the y direction at 3 cN yarn tension, whereas its displacement at yarn tension of 9 cN of was 1.65 and 0.6 mm in the x and y directions, respectively (see Figures 5(a1) and (b2)). According to the ANOVA evaluation, yarn tension was an important parameter on needle displacement in both the x and y directions at 95% significant level (x: F = 325.000, p = 0.000; y: F = 400.000, p = 0.000) (see Table 3). Meanwhile, for the machine speed of 20 rpm, the needle bounced over 1.3 mm in the x direction and 0.3 mm in the y direction when the yarn tension was 3 cN. However, at 9 cN, the displacement values were 1.5 mm (x direction) and 0.5 mm (y direction). Also for the machine speed of 20 rpm, ANOVA results showed that yarn tension affected the needle displacement in both x and y directions significantly (95% significant level x: F = 400.000, p = 0.000; y: F = 400.000, p = 0.000) (see Table 3). Knapton and Munden⁵ declared that nearly as much as one half of the loop length is lost to following needles at high input tensions. Also, under the condition of low input tension, the needle at the knitting point robs yarn from only one needle. When the input tension is increased, it then becomes possible for the needle at the knitting point to rob yarn from two needles until an input tension is reached where the yarn breaks out.²⁹ Our results complied with this high tension effect on robbing back because the velocity of the needle, when the yarn tension was high, was higher at robbing back, in comparison to that of the needle at low yarn tension (see Figure 5(c3)).

During the production with 100% ring cotton yarn with tension of 6 cN, it was found that there was a tendency to follow the cam path when the machine speed was increased from 15 to 20 rpm. At the speed of 15 rpm, the needle bounced to 1.7 mm in the x and 0.6 mm in the y direction. However, at 20 rpm the needle bounced to 1.5 mm in the x and 1.3 mm in the y direction (see Figures 6(a1) and (b2)). Also, due to the fact that the needle bounced less at high machine speed (20 rpm), surprisingly, this very needle had a slower velocity during robbing back (see Figure 6(c3)). As machine speed was increased, a considerable impact force in magnitude grows between the needle and the clearing cam.²⁵ Accordingly, when the needle contacted the clearing cam for the first time at the machine speed of 20 rpm, it appeared that a higher force occurred in comparison to the force at the machine speed of 15 rpm. As a result, the acceleration of the needle at 20 rpm was approximately 50% higher than that of the needle at 15 rpm (see Figure 6(d4)). Under the influence of this very impact force, the needles lost their contact with the cam, and as can be seen from the Figures 6(a1) and (b2), they hit the cam for the second time. Although the needles had higher acceleration at the machine speed of 20 rpm than the ones at 15 rpm when they did contact with the clearing cam for the first time, they had a similar acceleration characteristic at the second contact with the cam due to the needles bouncing less and hitting the cam sooner at the machine speed of 20 rpm (see Figure 6(d5)).

Figure 6.

Effect of machine velocity on needle displacement behavior during production with 100% ring cotton yarn.

The paired sample t-test between the data obtained from 15 and 20 rpm machine speeds revealed that the bouncing distances of the needle in both directions were statistically significant (95% significant level x: t = 22.000, p = 0.000; y: t = 5.000, p = 0.001) (see Table 3).

Textured polyester yarn

A comparative study of needle displacement at the knitting point for yarn tensions of 3 and 9 cN revealed that the effect of yarn tension increase on needle displacement was more significant than in the case of production with 100% cotton ring yarn. Irrespective of machine speed, there was no tendency for the needle to bounce at yarn tension of 3 cN (see Figure 7(a1)). However, when yarn tension was increased, for instance, from 3 to 6 cN, the needle did not follow the cam path and started to bounce. In the study, at 9 cN, the needle bounced to 1.8 mm in the x direction and 0.6 mm in the y direction when the machine speed was 15 rpm. However, at the machine speed of 20 rpm, it made a displacement of 1.5 mm in the x and of 0.4 mm in the y direction (see Figure 7(b2)). ANOVA evaluation of the data also suggested that yarn tension had an influence on the behavior of needle displacement in both the x and y directions (95% significant level, 15 rpm – x: F = 13075.00, p = 0.000; y: F = 2275.000, p = 0.000 and 20 rpm – x: F = 3100.000, p = 0.000; y: F = 1300.000, p = 0.000) when producing with 100% textured polyester yarn (see Table 3).

Figure 7.

Effect of yarn tension on needle displacement behavior and velocity at 20 rpm during production with texture polyester yarn.

The decrease of the yarn tensile force value refers back to yarn elongation, because there is a greater amount of yarn in the section that has to be elongated before the tensile force increases, as discussed by Matthes et al.³⁰ Hence, the reason why lower yarn tension did not cause the needle to bounce may be the high elongation property of the textured polyester yarn. Koo² suggested that an increase in the friction between the yarn and needle hook is produced by high tension and, hence, such a needle did not follow the cam thoroughly due to the high tension between the hook and the yarn. Also the literature on the mechanics of single jersey loop formation revealed that the phenomenon of robbing back plays an important role in determining loop length and, in particular, yarn input tension strongly affects robbing back, although it occurs even at zero input tension.¹ When the input tension is increased, it then becomes possible for the needle at the knitting point to rob yarn from two needles until an input tension is reached where the yarn breaks out.^5,16 This is because when yarn tension in the needle hook increases, it was difficult for the needle to rob yarn from its neighbor due to the opposite force at the needle hook. Similarly, in our study we found that the needle subjected to lower yarn tension had lower velocity during the robbing back (see Figure 7(c3)). This may suggest that higher tension increased the robbing back, which is in agreement with the literature.5,16

According to the literature survey,^1,21,23,24 the tendency for the needle to bounce on the cam is decreased as the rotational speed of the machine is increased. The data obtained for the textured polyester yarn showed that the needle displacement behavior was dependent on the yarn tension. For the yarn tensions such as 3 and 6 cN as machine speed increased, the needle bounced more in both x and y directions. For instance, for the textured polyester yarn whose tension was 6 cN, the needle made a displacement of 1.35 mm in the x direction and 0.3 mm in the y direction at 15 rpm, whereas it bounced to 1.6 mm horizontally and 0.5 mm vertically at 20 rpm (see Figures 8(a1) and (b2)). However it was surprising that, at high yarn tensions (i.e. 9 cN), the needle bounced more at low machine speed (15 rpm). As discussed earlier, the velocity of the needle at robbing back position was affected by needle bounce. Although at low yarn tensions they tended to follow the cam thoroughly, the needle at the machine speed of 20 rpm had a faster velocity during robbing back because of its bounce characteristic. This can be seen from Figure 8(c3). When the needle at the machine speed of 20 rpm contacted the clearing cam for the first time, a large reaction and impact forces took place than at the machine speed of 15 rpm. As a result, acceleration emerged more at this very needle (see Figure 8(d4)). Due to the fact that the place where they hit the cam second time were not the same, second force appeared were not likewise so an acceleration seen from Figure 8 D5 was come up.

Figure 8.

Effect of machine velocity on needle displacement behavior during production with 100% textured polyester yarn.

In addition, the effect of changing rotational speed of the machine on needle displacement behavior was not a statistically significant factor according to the paired t-test (x: t = –1.270, p = 0.240; y: t = –0.286, p = 0.782) (see Table 3).

Comparison of ring cotton and textured polyester yarn results

Yarn tension is a very important parameter on needle displacement because if it is high, the needle tends to be pulled up by this force. Yarn elongation is another parameter that affects yarn tension during the knitting process. If the yarn elongation property is high, it extends more so the force in the needle hook is less. Moreover, the coefficient of yarn/metal friction is as important as the elongation property of the yarn.¹⁶ During knitting, also the frictional coefficient influences the process when yarn is slipping from the hook as well as when it is wrapping the trucks of the needles or forming the loops.³¹

Cotton yarn has a rougher surface then textured polyester yarn. Textured synthetic yarns have a strong non-linear behavior as a result of this process.³⁰ In the study, the elongation of polyester is approximately three times higher than that of the cotton yarn (see Table 2). At the machine speed of 15 rpm the data showed that it was the ring cotton yarn that made the needle bounce more than the polyester yarn so far as low yarn tension values, such as 3 cN, were concerned (see Figures 9(a1) and (b2)). The paired sample t-test showed that the difference between the bouncing distances of the needles in x and y directions for the ring cotton and textured polyester yarns was statistically significant (95% significant level x: t = 2.403, p = 0.043; y: t = 3.965, p = 0.004) at the machine speed of 15 rpm (see Table 3). The higher bounce distance values of the needle working with relatively higher friction cotton yarn, when compared with the textured polyester yarn, may be explained such that at low yarn tension where the yarn was robbed from one needle only, it was difficult for the ring cotton knitting needle to rob yarn from its neighbor. As a result, during the robbing back position the ring cotton knitting needle had a more constant and higher velocity value compared to the textured polyester one (see Figure 9(c3)).

Figure 9.

Effect of fiber type during production with low yarn tension at 15 rpm.

If, however, machine speed and yarn tension were increased simultaneously (i.e. machine speed 20 rpm and yarn tension 9 cN), the behavior of needle displacement changed in such a way that irrespective of yarn type the needles made the same displacement in the x direction, whereas the needles working with the ring cotton yarn bounced higher in the y direction. A large impact force occurred when the needle butt hit the clearing cam for the first time and this impact was higher while working with higher machine speeds, such as 20 rpm. The positive effect of greater elongation percentage of the textured polyester yarn was significantly limited due to the combined influence of this very impact force together with high yarn tension. Also, the paired sample t-test for higher machine speeds and yarn tension values (e.g. 9 cN) supported this result that there was no significant difference between bouncing distance and height of the needles when they were knitted using different yarn types (95% significant level x: t = 1.109, p = 0.299; y: t = 0.555 p = 0.594) (see Table 3).

Effect of elastane

Ring cotton-elastane yarn

If the ring cotton yarn was plaited with elastane yarn, the needle tended to bounce more both in the horizontal and vertical directions at the machine speeds of 15 and 20 rpm. When the speed was 15 rpm and the cotton yarn tension was 6 cN, the needle bounced to 1.7 mm in the x and 0.6 mm in the y direction. However, when the yarn tensions were 6 cN cotton–9 cN elastane (i.e. elastane was included in knitting), the needle bounced to 1.9 mm in the x and 0.8 mm in the y direction. When the machine speed was increased up to 20 rpm, the needle working with 6 cN cotton yarn made a displacement of 1.5 mm in the x and 0.3 mm in the y direction, whereas its horizontal and vertical displacement values were 1.8 and 0.6 mm, respectively, for the case of 6 cN cotton–9 cN elastane yarn tensions (see Figures 10(a1) and (b2)). The resistance of the yarn to the loads depends upon viscoelastic properties of the yarn.³² Since knitting of elastane and cotton yarn together exhibits higher elasticity modulus, it offers higher resistance to the loads occurring in knitting. This results in more prominent bouncing behavior of the needle. Also, the independent t-test results suggested that elastane is a highly significant factor on needle movement for both x and y directions at the machine speed of not only 15 rpm but also 20 rpm (95% significant level 15 rpm x: t = –2.657, p = 0.012, y: t = –3.803, p = 0.001; 20 rpm – x: t = –5.586, p = 0.000, y: t = –5.617, p = 0.000) (see Table 3). In addition to that, the velocity of the needle producing 100% ring cotton yarn was slower during robbing back (see Figure 10(c3)). Ghosh and Banerjee¹ suggested that yarn input tension strongly effects robbing back and its increase also advances the robbing back. The cotton-elastane knitting needle had more pre-tension, and thereby it caused more robbing back at the stitch formation point. When the yarn tension increased, more yarn was robbed from one needle to another, which resulted in higher velocity at the needle.

Figure 10.

Effect of elastane on needle displacement behavior and velocity at 20 rpm during production with ring cotton yarn.

Furthermore, in order to investigate the impact of yarn pre-tension and the responding feeding load upon the needle movement, fabrics were manufactured at different elastane yarn tensions, namely 3, 6 and 9 cN. It was found that including elastane in the production increased the needle bounce for both machine speeds under discussion; however, changing the tension did not have as much influence on needle bounce behavior as was expected. The elongation percentage of elastane is very high so its force on the needle hook was not less than the force applied by the face yarn, that is, cotton yarn. As a result, the bouncing behavior of the needles working with the cotton yarns of the same tension were the same but elastane yarn tensions were different. Moreover, it was observed that when the elastane tension was kept constant while cotton yarn tension was increased, the higher the cotton yarn tension was, the more the needle tended to bounce.

When knitted with ring cotton-elastane yarn, the needles presented the same bouncing value in the y direction, whereas the displacement of the needle in the x direction was greater at the machine speed of 20 rpm. Knitting at 3 cN cotton–9 cN elastane yarn tensions, the needle bounced to 1.7 mm in the x and 0.6 mm in the y direction at the machine speed of 15 rpm. However, at 20 rpm, its displacement in the x direction was higher (see Figures 11(a1) and (b2)). The literature survey showed that as the machine speed is increased, the impact force magnitude also increases, the period of time during which the needle contacted the cam is shortened and both the number and duration of the bounces increases.^1,21,23,24 The results were in agreement with the findings of the literature. However, according to the paired sample t-test conducted between the needle displacement values obtained for 15 and 20 rpm machine speeds, the machine speed was not a statistically significant factor, providing that elastane was employed in the production (95% significant level x: t = –1.490, p = 0.148; y: t = 1.199, p = 0.241) (see Table 3). Moreover, the results suggested that there was no remarkable difference between the displacement values of the needles depending on different elastane yarn tensions. However, the findings suggested that the bouncing behavior of the needle had an important effect on the velocity of the needle throughout the robbing back. While the yarn was robbed back, the more the needle bounced, the higher its velocity was (see Figure 11(c3)). After downward movement though the stitch cam, the needle hit the clearing cam and then it bounced through this very cam with the help of impact and reactive forces. In this example, by the agency of its high velocity, the needle at the machine speed of 20 rpm hit the cam faster and huge acceleration occurred (see Figure 11(d4)). After this hit movement, the needle butt stroked the clearing cam second time. As the needle at the machine speed of 20 rpm bounced longer comparing to the ones at 15 rpm, this very needle hitting the cam was more influential so its acceleration was high at this second hit (see Figure 11(d5)).

Figure 11.

Effect of machine velocity on needle displacement behavior during production with ring cotton-elastane yarn.

Also, it was found that changing elastane yarn tension had no statistically significant effect on needle displacement for the either machine speeds.

Textured polyester-elastane yarn

As far as textured polyester yarn was concerned, the effect of addition of the elastane to the production was more apparent when working with low yarn tension (e.g. 3 cN), whereas its presence was not effective at high polyester yarn tensions (e.g. 6, 9 cN) at the machine speed of 15 rpm. In particular, working at low machine speeds, such as 15 rpm, it was found that elastane was not an influential parameter for needle movement when textured polyester yarn was employed. Also, the independent t-tests conducted between needle displacement values of the needles working with 100% textured polyester and “textured polyester-elastane” presented that there was a statistically significant difference between these two groups at low yarn tensions (e.g. 3 cN) (95% significant level x: t = –12.134, p = 0.000; y: t = –5.400, p = 0.001), whereas no statistical difference was found at high yarn tensions (x: t = –0.884, p = 0.386, y: t = –0.844, p = 0.408) (see Table 3). On the other hand, at the machine speed of 20 rpm, irrespective of the tension of the textured polyester yarn, the needles bounced more when the elastane yarn was included in the knitting process. Moreover, the needle bouncing was more evident as the tension of textured polyester yarn was increased. For instance, the needle working with 9 cN textured polyester yarn bounced to 1.5 mm in the x and 0.4 mm in the y direction, whereas the one knitted with 9 cN polyester–9 cN elastane made a displacement of 1.8 mm in the x and of 0.6 mm in the y directions (see Figures 12(a1) and (b2)). Both textured polyester and elastane yarns show high elongation at break (see Table 2). As a result, they force the needle more, which in turn causes it to bounce more. Also, with reference to the independent t-test, the difference between the displacement values in both directions, that is, x and y, of the needles working with 100% textured polyester and textured polyester-elastane was statically significant only for the displacement in the x direction (95% significant level x: t = –2.121, p = 0.041), whereas no significant difference was found for the bouncing values of the needles in the y direction (95% significant level y: t = –1.315, p = 0.197) (see Table 3). Also, the study showed that when polyester yarn tension was kept constant, there was no difference in the needle displacement, depending on the increase in elastane yarn tension. In addition to these findings, increasing polyester yarn tension, by keeping elastane yarn tension constant, forced the needle to bounce more. Finally, the needle gave higher velocity values during robbing back when elastane-textured polyester yarn was utilized (see Figure 12(c3)).

Figure 12.

Effect of elastane on needle displacement behavior and velocity at 20 rpm during production with textured polyester yarn.

When the effect of machine speed on needle displacement behavior of the needle was evaluated, it was found that for low textured polyester yarn tensions increasing machine speed caused the needle to bounce more in both directions. For instance, at the machine speed of 15 rpm, the needle working with 3 cN textured polyester–3 cN elastane yarn bounced to 1.0 mm in the x and 0.1 mm in the y directions, whereas the one knitting at the machine speed of 20 rpm made a displacement of 1.5 mm in the x and 0.3 mm in the y directions (see Figures 13(a1) and (b2)). However, according to the paired t-test results, machine speed was not an influential parameter on needle displacement behaviors at different machine speeds (x: t = 1.674, p = 0.106; y: t = 0.721, p = 0.477) (see Table 3).

Figure 13.

Effect of machine velocity on needle displacement behavior during production with textured polyester-elastane yarn.

Depending on the machine speed, the velocity behavior of the needle during robbing back was investigated and it was found that the needle at the machine speed of 20 rpm was faster in that zone compared to those at 15 rpm (see Figure 13(c3)). When the needle contacted the clearing cam for the first time at the machine speed of 20 rpm, it appeared that a higher force occurred in comparison to the force at the machine speed of 15 rpm. As a result, the acceleration of the needle at the machine speed of 20 rpm was approximately 50% higher than that of the needle at 15 rpm (see Figure 13(d4)). Under the influence of this very impact force, the needle at the machine speed of 20 rpm bounced for the second time and hit the cam again so second acceleration took place (see Figure 13(d5)). However, at the machine speed of 15 rpm, the needles tended to follow the cam path and second acceleration did not arise.

Comparison of ring cotton-elastane and textured polyester-elastane yarn results

When the elastane yarn was fed together with the ring cotton or textured polyester yarn to the needles, the yarn force opposite needle movement increased, and therefore the needles tended to bounce more when compared with the results of the section titled Effect of yarn tension and machine speed. However, the needle movement was somewhat different in the case of working with different yarns, such that at low yarn tension and at low machine speed, it was the cotton-elastane yarn that influenced the needle bounce values more. The needle knitting with the cotton yarn of 3 cN tension and with the elastane yarn of 6 cN tension bounced to 1.7 mm in the x and 0.6 mm in the y directions. On the other hand, the textured polyester-elastane one, at the same yarn tension values, caused the needle to lose its contact with the clearing cam up to 1.3 mm in the x and 0.3 mm in the y directions (see Figures 14(a1) and (b2)). Also, the paired sample t-test supported this finding in such a way that there was a statistically significant difference between the needle displacement in the x and y directions while working with low yarn tension and low machine speed of 15 rpm (95% significant level x: t = 9.827, p = 0.000; y: t = 11.000, p = 0.000) (see Table 3). This may be due to the fact that low yarn tension, in combination with higher elongation percentage of polyester yarn, did not have a big influence on pulling up the needle. Also, the velocity of the needle knitting with cotton-elastane yarns was faster during the robbing back (see Figure 14(c3)).

Figure 14.

Effect of fiber type and elastane at 15 rpm.

When both machine speed and yarn tension were increased, the needles working with both ring cotton-elastane and textured polyester-elastane yarns showed similar bounce behavior. Higher yarn tension lifts the needles up more, and high machine speed made needles tend to bouncing more. With the combination of these two factors, elongation and smooth surface properties of textured polyester yarn appeared not to be influential parameters, like yarn tension and machine speed. In brief, both of the needles bounced to the same distance in the x direction but the needle producing cotton-elastane yarn had greater displacement in the y direction.

Summary

The needle dynamics on an industrial circular knitting machine was studied using a high-speed camera. The important results of the study under discussion can be summarized as follows.

The needle displacement in both x and y coordination did tend to increase as the yarn tension increased, irrespective of machine speed.

For all the relevant cases discussed, the presence of elastane yarn was a significant parameter on needle movement. However, its input tension was not as influential as its presence.

The data obtained for knitting with textured polyester yarn showed that the needle movement was highly dependent on yarn tension.

Producing with cotton ring yarn made the needle bounce more than knitting with the textured polyester yarn, especially for low yarn tension and machine speed.

For all situations, the velocity of the needle was higher at robbing back, which in turn caused a greater displacement.

Accordingly, the following may be suggested that for a longer needle life:

manufacturing optimized yarn would decrease the force inside needle hook;

working at lower machine speeds causes needles to bounce less, which in turn results in decreasing impact force;

working with yarns of low frictional properties would allow needles to follow the cam path thoroughly, which results in less abrasion on the cam and needle butt.

Finally, due to the novelties of the work it may be concluded that the study was the very first approach in which real-time measurements regarding needle displacement within a cam track were taken for an industrial-type circular knitting machine thanks to the use of a high-speed camera. In addition to that, unlike the literature in which mostly cotton was used as a material, and which has produced some findings for this very material, in this work the effect of different materials, such as textured polyester as well as elastane yarns, on the needle displacement was discussed for an industrial-type circular knitting machine, the important results of which were given above.

Footnotes

Funding

This work was supported by the Turkish Higher Education Council under the framework of the PhD Research Support Program.

Acknowledgment

We would like to express our sincere gratitude to Dr Achim Hehl from RWTH Aachen University Institut für Textiltechnik (ITA) for his kind and valuable support.

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